24 EOCENE MOLLUSC A. 



Spec. CJiar. 0. testa magna, crassissimd, ovato -circular I, inaquivalvi, irregulariler 

 lamellosd ; valvd inferior e gibbosd, convexd ; valvd superior e plana, impressione muscidari 

 sublunari, profunda ; umbonibus brevibus; cardine plano-triaugulari, striato, foveold 

 profunda ; marginibus superue rugoso-plicatis. 



Shell large and thick, ovately circular, inequivalved, irregularly lamellated ; inferior 

 valve gibbous and convex ; upper valve flat ; muscle-mark gibbosely lunate ; beaks short, 

 hinge area flattened and triangular, deeply marked with lines of growth ; rugosely plicated 

 on each side of the hinge. 



Diameter, 6 inches. 



Localities. Bognor, Barton {Edwards), Cuffell near Basingstoke {Prestwic/t). 



Belgium, Kleyn Spauwen ; Pietrebais, pres de Chapelle St. Laurent 

 {Ngst) ; Middle Limburg {Lyell). 



France, Chaumont, Valmondois {Deshayes). 



M. Bronn (Leth. Geogn., b. 11, p. 355) has considered the fossil found in the Crimea, 

 called gigantea by MM. De Verneuil and Deshayes, as a distinct species, to which he has given 

 the name of 0. Pgrenaica ; and he is not alone in this opinion. If the figure by M. Deshayes 

 represents its constant character, the muscle-mark appears to be not only of a different 

 form, but to be situated nearer to the hinge than in our specimens. Form, in this genus, 

 is of itself a character by no means to be depended upon, as the shell is frequently 

 distorted. 



Fig. 6, Tab. VIII, appears to me to possess the characters required for a place in this 

 species. The principal difference is in the shape ; the lengthened beak, and elevated muscle- 

 mark, are consequent upon its peculiar form ; but the cicatrice is of a different colour from 

 the shell, as is usual in gigantea. The rugosities, or denticulations, at the shoulders which 

 distinguish this species are not very evident, although there are traces of them, and they 

 are perhaps destroyed. 



Mr. Sowerby, in Dixon's ' Geology of Sussex,' pp. 95 and 173, introduces an oyster under 

 the specific name elep/tantopus ; and, after giving a full description, but no figure, says, 

 '•' This differs from gigantea in the form of the hinge-pit, which is considerably elevated in 

 that species, and in the depth of the hollow valve. It shows the same cellular tissue as 

 0. cariosa of the Bognor Rock, which may possibly be the young of the same species, 

 although such large specimens have not been found at that place." 



The specimen to which the name elephantopus is attached in the late Mr. Dixon's collec- 

 tion in the British Museum is an upper or right valve of great solidity; but it does not appear 

 to me to offer any character that will separate it from gigantea, to which I believe it belongs. 

 The peculiarity of the hinge in extending inwards is the result of the excessive thickening 

 of the shell, by the successive layers of calcareous matter over the entire surface of the 

 interior. Similar specimens are in the cabinets of Mr. Edwards and Dr. Bowerbank, and 

 I am unable to detect any other difference than that which has been produced by age. 

 These shells possess the same peculiar structure, as is so strikingly displayed in some 



